Stressing device

ABSTRACT

The device comprises a tubular body portion (1) in which are fitted first and second jaws (2, 4) for holding a specimen (3) to be tested. A compression spring (5) or pressure chamber (29), are housed within the body portion (1) and act to urge the first jaw (2) outwards whilst the second jaw (4) is secured to the body portion thereby applying stress to the specimen (3). The device is sufficiently small to enable it to be used in confined spaces, such as the underside of a vehicle or within a vehicle engine compartment, so the tests can be carried out in real-life environments. If hydraulic actuation of the pressure chamber (29) is used, varying stresses can be applied to the specimen (3) which are also dependent upon real-life movements and/or stresses.

This application is a continuation of PCT/GB91/01386 filed Aug. 15,1991.

TECHNICAL FIELD

This invention relates to a stressing device for use, for instance, intesting the durability of bonded joints.

BACKGROUND ART

The durability of structural adhesive bonds exposed to combinations ofstress and an aggresive environment has traditionally been evaluatedusing large, bulky stressing devices suitable only for use in laboratorybased or static outdoor test environments. These devices may typicallybe a meter or more in length and 5 to 10 cm in width.

DISCLOSURE OF INVENTION

The present invention aims to provide a smaller stressing devicesuitable for use in confined spaces, e.g. the underside of a vehicle orinside a vehicle engine compartment, to enable durability tests to beconveniently carried out in real-life environments and over extendedperiods of time.

According to a first aspect of the invention there is provided astressing device for applying a stress to a specimen being tested, thedevice comprising a tubular body portion having a bore passing from oneend of the device to the other; first and second jaws mounted within thetubular body portion and arranged to be secured to opposite ends of aspecimen to be tested; securing means for securing the first jaw to thebody portion; and resilient means housed in a chamber within the bore ofthe body portion and arranged to apply a load to the second jaw and thusto the specimen being tested, at least one cut-out being provided in theside of the body portion to provide access to a specimen held betweenthe first and second jaws and to expose the specimen to the surroundingenvironment, the device being sufficiently small to enable it to be usedin confined spaces, such as the underside of a vehicle or within avehicle engine compartment, so that tests can be carried out inreal-life environments.

According to a second aspect of the invention, there is provided amethod of testing a specimen using a stressing device as detailed abovecomprising the steps of: fitting the specimen within the device andinstalling the device in a real-life situation, e.g. by mounting it onthe body of a vehicle, so it is subject to real-life temperature changesand a real-life chemical environment.

Preferred features of the invention will be apparent from the followingdescription and the subsidiary claims of the specification.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be further described, merely by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional side view of a first embodiment of astressing device according to the invention and also shows an adaptorfor mounting the device within a tensometer;

FIG. 2 is a plan view of a body portion of the device shown in FIG. 1;

FIG. 3 shows plan views of specimen jaws used in the device shown inFIG. 1 and a nut which fits into one end of the device;

FIG. 4 is a side view of a second embodiment of a stressing deviceaccording ot the invention showing two possible modifications to thefirst embodiment;

FIG. 5 is a schematic view of one end of a stressing device illustratinga further modification of the embodiments shown in the precedingdrawings; and

FIGS. 6A and 6B are schematic diagrams of two forms of movement sensorwhich may be used in conjunction with the device shown in FIG. 5.

BEST MODE OF CARRYING OUT THE INVENTION

The device illustrated in FIGS. 1 to 3 comprises a substantially tubularbody portion 1, a first jaw 2 for holding one end of a specimen 3, asecond jaw 4 for holding the other end of the specimen 3, a compressionspring 5, a nut 6 which fits into one end of the body portion 1 and asecurity bolt 7 which fits into the other end of the body portion 1.

In use, the components of the device are assembled as shown in FIG. 1with the jaws 2 and 4 fitted within a bore 8 of the tubular bodyportion 1. The specimen 3 is secured between the jaw 2 and 4 by means ofstainless steel pins (not shown) fitted through holes 2A and 4A providedin the ends of the jaws 2 and 4. The compression spring 5 is fittedwithin a spring chamber comprising an enlarged portion 9 of the bore 8and acts between a shoulder 10 at the inner end of the spring chamber 9and the nut 6 which is secured to the outer end of the jaw 2. The nut 6is designed to be a sliding fit within the spring chamber 9 of the bore8 and is screwed onto the outer end of the jaw 2.

The outer end of the jaw 4 is provided with a threaded fixing forreceiving the security bolt 7 and for attaching it to one side of atensometer (as will be described further below).

The device illustrated in FIGS. 1 to 3 is designed so that a specimen 3can be accurately stressed without the need to calibrate the spring 5.This is achieved by mounting the device within a conventionaltensometer, applying the required stress and then clamping the jaw 4 inplace by means of grub screws 11. To do this, the device is firstsecured to a special adaptor 12 by means of bolts (not shown) secured toholes 13 and 14 provided in the body portion 1. One side of thetensometer is then attached to the adaptor 12 and the other side to theouter end of the jaw 4.

The spring 5 and nut 6 are loosely fitted into the end of the bodyportion 1 and the tensometer crosshead is then moved until the specimen3 is approximately centred. The exact load required is then imposed onthe specimen 3 by turning the nut 6 and/or moving the tensometercrosshead in such a manner that the specimen 3 remains approximatelycentered.

The adaptor 12 comprises a rigid bar which is preferably secured to thebody portion 1 in two places, i.e. at the holes 13 and 14, to helpensure the device is kept in alignment with the tensometer. The adaptor12 is designed so that the line between the two sides of the tensometerpasses through the center line of the body portion 1 to ensure jaw 4 canfreely slide within the body portion 1 as the stress is applied.

As tension is applied between the jaw 4 and the body portion 1, the jaw4, specimen 3 and jaw 2 are put under tension and the spring 5 iscompressed. When the desired stress has been applied, the jaw 4 islocked in position with respect to the body portion 1 by the grub screws11 so the spring 5 maintains the set stress on the specimen 3. Thearrangement shown has four grub screws 11 at right angles to each other,the fourth being fitted into the hole 14 once the adaptor 12 has beendisconnected. The grub screws 11 also serve to retain the jaw 4 withinthe device should the sample 3 fail. The jaw 2 is held captive withinthe device by the shape of its inner end and the nut 6 secured to itsouter end.

Once the desired stress has been set in this way, the device may beremoved from the tensometer, detached from the adaptor 12 and thesecurity bolt 7 fitted into the outer end of the jaw 4. The securitybolt 7 is provided as a precaution against slippage between the jaw 4and the grub screws 11 during handling and exposure of the device. Thebolt 7 is screwed into the end of the jaw 4 until its flange 7A isseated within a recess 16 provided in the end of the body portion 1. Thedevice can then be mounted in the required environment, e.g. theunderside of a vehicle, the holes 13 and 14 being used to secure thedevice in place.

The tubular body portion 1 may typically be formed from an anodisedaluminum alloy (although stainless steel may also be used) and wouldtypically be around 5 to 20 cm in length and have an external diameterof around 1 to 3 cm. The particular device illustrated in FIGS. 1 to 3is approximately 14 cm long and has an external diameter ofapproximately 2.5 cm. A large slot 15 or cut-out is provided in one orboth sides of the body portion 1 so the sample being tested is exposedto the ambient conditions and is clearly visible. Holes 17 (see FIG. 2)are also provided through the sides of the body portion 1 to facilitatethe attachment of a sample 3 to the jaws 2 and 4. The steel pins (notshown) used to attach the sample 3 to the jaws 2 and 4 may be passedthrough these holes 17 so they can then be fitted into the holesprovided in the inner ends of the jaws 2 and 4.

The jaws 2 and 4 may be formed of stainless steel. The inner ends of thejaws 2 and 4 which are attached to the sample being tested may have avariety of forms for securing to a range of different specimenconfigurations. A number of different forms of jaws may be provided withthe device to enable different types of specimens to be held therein.

The jaws 2 and 4 and the corresponding portions of the bore 8 may have anon-circular section, e.g. square or rectangular, to prevent rotation ofthe jaw 2 and 4 within the body portion 1.

The security nut 7 may also be replaced by some other form of device forpreventing movement of the jaw 4 with respect to the body portion 1 oncethe required load has been applied to the specimen 3, such as anexternal locking ring (not shown) which screws onto the end of the jaw 4projecting from the body portion 1 or a transducer (described furtherbelow)

During exposure, dimensional changes within the specimen 3 may occurproducing corresponding changes in the stress applied thereto. Thespring 5 is preferably selected to minimise sensitivity to thesedimensional changes and to maximise sensitivity for setting the load byselecting the lightest, i.e. weakest, spring consistent with the load tobe applied. Typical spring rates suitable for bonded lap shear joints ofaluminium substrates would be around 50-300 N/mm. However, the devicecan be used with a wide range of spring strengths and is capable ofapplying very high loads to a sample, e.g. up to 3 KN.

As the spring is encased within the body portion 1, it is reasonablyprotected from the surrounding environment. However, in some cases, itmay be desirable to seal the spring chamber 9 by providing appropriateO-ring seals at each end of the chamber, e.g. in the manner shown inFIG. 5 (described below). Electroless nickel plated springs made ofchrome-vanadium steel have been found to be particularly suitable interms of strength and corrosion resistance.

The assembled device may typically have a weight in the range 50 g to250 g, for instance around 150 g. This compares with conventional,laboratory stressing devices having a weight of the order of 2 g to 7kg.

A number of modifications and refinements may be provided for the basicversion of the stressing device described above.

As shown in FIG. 4, sensing means may be provided to permit remotedetection of failure of the specimen 3 and these are preferably providedwithin a sealed chamber to protect them from the environment.

In the arrangement illustrated, the sensing means comprises a electricalswitch 20 which is normally open but which is closed when the specimen 3under test fails. The body portion 1 in FIG. 4 is similar to that shownin the preceding Figures but is made slightly longer to accommodate theswitch 20. The switch 20 comprises two stainless steel terminals 21A and21B in the form of small bolts which are mounted in a block 22 ofinsulating material such as Tufnol (trade name for a fibre reinforcedplastics material). The block is mounted within a recess in the end ofthe body portion 1 and held in place by a circlip 23. A phosphor bronzecontact strip 24 is secured to the inner end of one of the terminals 21Aand extends across the space between the end of the first jaw 2 and theinner end of the other terminal 21B. If the specimen 3 fails, the firstjaw 2 is forced outwards by the pressure of the spring 5 on the nut 6 sothe end of the first jaw 2 engages the phosphor bronze contact 24 andpushes this into contact with the terminal 21B thus closing the switch.

The closure of the switch 20 may be sensed by a monitoring device (notshown) remote from the stressing device but electrically connected tothe terminals 21A and 21B. A large number of stressing devices, e.g. 100or 200, mounted on a structure such as a vehicle may all be connected tothe same monitoring device and this can be arranged to indicate when anyspecimen fails and in which device it is fitted.

The switch contacts are housed within the body portion 1 of the deviceso are protected from the outside environment. However, as additionalsecurity, O-ring seals 25 and 26 may be provided between the block 22and the body portion 1 and between the first jaw 2 and the body portion1, respectively, as shown to prevent ingress of moisture etc. whichmight cause the electrical contacts of the switch to corrode.

The arrangement described above provides a simple, robust and compactsensing device. However, other suitable switch arrangements for sensingfailure of a specimen will be apparent to those skilled in the art.

FIG. 4 also shows a transducer 27 fitted to the outer end of the secondjaw 4. The transducer 27 may comprise strain gauges or other means, e.g.piezoelectric devices, for measuring the stress applied to the specimen3. With such an arrangement, the grub screws 11 and security bolt 7 arenot required to secure the second jaw 4 relative to the body portion 1.Instead, the second jaw 4 is secured to the body portion 1 via thetransducer 27. In the arrangement illustrated, a bolt 28 secures theouter end of the second jaw 4 to the transducer 27 which is thuseffectively compressed between the bolt 28 and the outer end of the bodyportion 1 by the stress applied to the specimen 3.

This arrangement is particularly suitable where it is desired to monitorvariations in the stress applied to the specimen 3, e.g. due totemperature variations and hence dimensional changes of the specimen orof components of the stressing device. The transducer 27 allows thesecond jaw 4 to move slightly with respect to the body portion as thestresses vary but these movements will be very small so the second jaw 4may still be regarded as being effectively secured to the body portion1.

Various possible constructions for the transducer will be apparent tothe person skilled in the art so will not be described. It will also beappreciated that such a transducer may be used with any embodiment ofthe device, including those shown in the other Figures, and can be usedindependently of the switch 20 described above.

The transducer may be used for continuous or intermittent monitoring ofthe stress applied to the specimen and, like the switch 20, may beconnected to a remote monitoring device (not shown). It may also be usedfor measuring the load applied when the device is set up rather thanusing a tensometer to measure the load as described above.

In the arrangement shown in FIG. 5, a pressure chamber 29 is used as theresilient means for applying stress to a specimen 3 in place of thespring 5 (although the resilient means may also comprise a pressurechamber with a compression spring mounted therein).

The pressure chamber 29 is formed within the chamber 9 in the bodyportion with seals 30A and 30B at each end to seal it from the externalenvironment. Stress is applied to the specimen 3 by pressurising thechamber 29 with hydraulic fluid, via a port 31, from an external source.The pressure within the chamber 29 acts against the seal 30A to urge thenut 6 and hence the first jaw 2 outwards in the same manner as thecompression spring described in the earlier embodiments. Otherarrangements for sealing the chamber 29 will be apparent to thoseskilled in the art. The pressure chamber may also be in the form of adouble acting piston arrangement so both compressive and tansilestresses can be applied to the specimen 3.

One benefit of stressing the specimen in this way is that the pressurewithin the chamber 29 can be arranged to vary in dependence uponreal-life conditions such as the movements of a vehicle to which thedevice is attached. Pressure may, for instance, be applied to thechamber 29, via hydraulic lines and the port 31, by a movement or stresssensor which senses movements or stresses applied to a structure.

Cyclic stresses may thus be applied to the specimen 3 and these may varyrandomly in dependence upon movements or oscillations of real-lifeconditions experienced by a structure, such as a vehicle body, as it isused. The pressure in the chamber 29 may vary from zero (or evennegative pressures) up to 1000 psi (6900 KN/m²) or even up to 2000 psi(13.8 KN/m²).

FIGS. 6A and 6B illustrate two forms of a movement sensor which may beused to actuate the hydraulic pressure chamber 29.

In FIG. 6A a vertically oscillating mass 32 is pivotably mounted to astructure 33 such as a vehicle body so as to be free to oscillatevertically in dependence upon the vehicle movements. Verticaloscillation of the mass 32 is transferred by a lever mechanism 34 tomovement of a piston within a hydraulic master cylinder or hydraulicpressure intensifier 35. Movement of the piston and the action of themaster cylinder or pressure intensifier 35 produces pressure variationsin a hydraulic pipe 36 which can be connected to the port 31 of one ormore stressing devices.

In FIG. 6B, a master cylinder or pressure intensifier 35 is connectedbetween a vehicle body 37 and a wheel axle 38 so as to sense directlythe relative movement of the axle 38 with respect to the body 37. Again,the pressure variations produced are transmitted to the pressure chamber29 of one or more stressing devices by a hydraulic pipe 36.

A wide variety of other sensors for sensing the movement or stressesapplied to a structure and providing a pressure signal to the stressingdevice which varies in dependence upon these real-life variations can,of course, be used in place of those described above. The system mayalso be refined, as desired, by the use of pressure intensifiers and/orby buffering or damping the pressure variations depending on therequirements of the test. If the stressing device were used in thelaboratory, these might, for instance, include arrangements employingmotor driven cams and pistons etc.

It will be appreciated that very small volume changes of within thepressure chamber 29 are required to produce relatively large changes inthe stress applied to a specimen 4 held within the device. With theappropriate use of pressure intensifiers, it is therefore feasible touse a single movement or stress sensor to provide the same varyingpressure signals to a large number of stressing devices.

So, by using hydraulic activation, it is possible to apply the samevariable stresses to a large number of specimens. A single source ofhydraulic pressure may, for instance, be applied to the pressurechambers of one hundred or more stressing devices, either in the fieldor in the laboratory, so that the specimen mounted in each deviceexperiences the same stresses. This would be very difficult to achieveusing any other arrangement, e.g. mechanical system transferring varyingloads by means of cams, levers, pistons etc.

A transducer such as that described in relation to FIG. 4 may, ofcourse, be used to measure and monitor the stress applied to thespecimen in a device using hydraulic actuating means as described above.

In addition to the modifications described above, an extensometer mayalso be incorporated in the device for creep observations.

It may also be desirable to use an anti-galvanic arrangement in whichthe sample 3 is electronically isolated from the remainder of the deviceand from the structure on which the device is mounted, e.g. by the useof appropriate insulating inserts and bushes.

As mentioned above, the device is designed to be very small andlightweight so it can be used in natural environments where space islimited and without major inconvenience. It is particularly suitable formounting on the underside of a vehicle, e.g. for testing the durabilityof bonded joints, and in such a situation it may need to be left inplace for at least a year so that the sample is exposed to the fullcycle of weather and environmental conditions in spring, summer, autumnand winter.

With the arrangements described in FIGS. 1 to 3, the required stress canbe pre-set in the laboratory using a conventional tensometer. The testloads can thus be accurately set and checked. This avoids the need tocalibrate the spring of each individual device. However, the spring maystill be calibrated in the conventional manner should this be desired. Atransducer such as that described in relation to FIG. 4 may be used tomeasure the applied stress in other arrangements.

The device has a wide range of possible applications, including:

adhesive bond durability testing

stress corrosion studies on metals

creep studies, e.g. of metals, polymers and composites

environmental stress-cracking of polymers

It is difficult to simulate real-life environments in the laboratory andthe stressing device described above helps overcome this. First, it issmall enough to be conveniently used in real-life situations, e.g.attached to the body of a vehicle, so it is exposed to the sameenvironment as the vehicle, i.e. it experiences the same temperaturechanges and chemical environment (rain etc.). In addition, with thearrangement shown in FIG. 5, rather than applying a fixed, pre-setstress to the specimen, it can be subjected to varying stresses whichare dependent upon the randomly varying stresses experienced by thestructure on which the device is mounted.

Although designed for use in the field, the device may also be used inthe laboratory where its small size enables the cabinet space requiredfor multiple tests to be greatly reduced compared to conventionaldevices. Its simple structure and design also enable it to beconsiderably less expensive to manufacture than conventional devices.

The device described above is designed to be of simple construction withas few parts as necessary. The body portion 1 is shaped to house all thenecessary components and is provided with the appropriate recesses,bores and holes to hold the components accurately in place. Thisintegrated design of the body portion 1 enables the device to be mademuch smaller than conventional stressing devices and with the minimumnumber of components.

Despite its small size, the device can however be used for the majorityof stress tests conventionally carried out. The device is capable ofholding a British Standard lap joint which is 1 inch (2.54 cm) wide witha 1/2 inch (1.25 cm) overlap (although it may be necessary to reduce thelength of the sample).

As indicated above, the device can be modified or adapted in a varietyof ways so despite its simplicity it is also very versatile.

The device is designed mainly to house single samples rather than aplurality of samples connected in series as used in conventionaldevices. However, it is in many cases more convenient to use a pluralityof such small devices each housing a single sample rather than one largedevice housing a plurality of samples. A plurality of small samples may,nevertheless, be connected in series and housed within the illustrateddevice if desired.

INDUSTRIAL APPLICABILITY

The stressing device may be manufactured and used in a wide variety oftests in which a specimen is to be stressed, whether in the laboratoryor in real-life situations.

I claim:
 1. A stressing device for applying a stress to a specimen beingtested, the device comprising:a tubular body portion having a borepassing from one end of the device to the other; first and second jawsmounted within the tubular body portion and arranged to be secured toopposite ends of a specimen to be tested; securing means for securingthe first jaw to the body portion; and resilient means housed in achamber within the bore of the body portion and arranged to apply a loadto the second jaw and thus to the specimen being tested, wherein atleast one cut-out is provided in the side of the body portion to provideaccess to a specimen held between the first and second jaws and toexpose the specimen to the surrounding environment, and wherein furtherthe first and second jaws are shaped and are provided with attachmentssuch that, once assembled within the body portion, they are held captiveand cannot become separated from the body portion, even when a specimenfails, without removing the said attachments, the device beingsufficiently small to enable it to be used in confined spaces so thattests can be carried out in real-life environments.
 2. A device asclaimed in claim 1 in which the securing means comprises at least onebolt for rigidly securing the second jaw relative to the body portion.3. A device as claimed in claim 1 in which the resilient means comprisesa compression spring, said stressing device further comprising a meansfor mounting said stressing device within a conventional tensometerwhich can apply a pre-set load to the specimen such that the securingmeans can secure the second jaw relative to the body portion so as tomaintain the load on the specimen when the device is removed from thetensometer.
 4. A device as claimed in claim 1 in which the resilientmeans comprises a pressure chamber connected by a hydraulic pipe to amovement or stress sensor arranged to provide varying pressures withinthe hydraulic pipe in dependence upon movement or stresses experiencedby a structure to which the movement or stress sensor is mounted.
 5. Adevice as claimed in claim 1 in which the resilient means comprises acompression spring, whereby the resilient means acts between the bodyportion and attachment means attached to the outer end of the second jawso as to apply tension to a specimen held between the first and secondjaws.
 6. A method of testing a specimen using a stressing device asclaimed in claim 1 comprising the steps of: fitting the specimen withinthe device and installing the device in a real-life situation so it issubjected to real-life temperature changes and a real-life chemicalenvironment.
 7. A method as claimed in claim 6 in which varying stressesare applied to the specimen by hydraulic pressure means within thedevice, the hydraulic pressure means being actuated by movement sensingmeans subject to real-life movements, whereby the stresses applied tothe specimen vary in dependence upon the real-life movements experiencedby the sensing means.